Design and Construction of a Microcontroller- Based ac Power Control System Akinkuade S. T Science Technology Department, Federal Polytechnic, Ado-Ekiti, Nigeria Itakorode O. D Science Technology Department, Federal Polytechnic, Ado-Ekiti, Nigeria Abstract—One of the numerous ways through which sophistication can be introduced into an electrical system is by providing users with some form of electronic power control; so that the level of illumination in a room, speed of food mixers, electric drills, sewing machines and heat generated by a heating element in an oven or incubators can be controlled with better accuracy and efficiency. The use of RC network in controlling the firing angle of a Silicon Controlled Rectifier(SCR) and Triac in power control circuits is limited to °, an attempt to increase the angle to 18° makes circuits to be complex and less reliable. The quest for possibility of controlling the ac power that is delivered to a load in the entire period of each half-cycle of a sinusoidal voltage prompts the use of a microcontroller in this work. A microcontroller; AT 80C52 was used to adjust the firing angle of a Triac; BT 136-600E in ten equal steps within each half-cycle of the mains ac voltage (220 V, 50Hz). The results indicate that firing angles between ° and 18° can be obtained using the circuit. Keywords—System; Control; Power; Firing Angle; Triac; Microcontroller. I. INTRODUCTION It is often desired to control the power fed to a load using electronic methods. Such methods permit a fine control of power with better efficiency than electrical methods. Semiconductor devices that are often used to control the flow of current in a circuit are diodes, diode ac (Diac) Thyristors or Silicon Controlled Rectifier (SCR) and Triode ac (Triac) [5]. Diodes only conduct when it is forward biased and have no external control of the start of conduction. Thyristors allow control of the start of conduction in the positive half-cycle of an ac voltage but rely on periodic reversal of current to turn them off. Triac on the other hand has the ability to conduct current in both half- cycles by using positive or negative gate pulse which provides control on the start of conduction; therefore it can be used to provide a control of power in ac circuits of lighting equipment, hot-air oven, electric incubator and electric heater and in universal single phase ac motor. In these systems, power is controlled by means of phase angle variation of the conduction period through the setting of different firing times corresponding to different firing angles. The mains voltage is an alternating type that reverses its polarity many times per second and it is sinusoidal in nature. The instantaneous value of such a voltage is usually expressed as: v(t) =Vsinωt (1) Where V is the amplitude of the voltage in volts and ω is the angular frequency in radians per second and t is time in seconds, the period ‘T’ which is the time it takes the voltage to complete a whole cycle can be written in terms of angular frequency as: T=2π/ω (2) The physical frequency f which is measured in hertz (Hz) is related to the angular frequency as: ω = 2πf (3) Therefore, T=1/f (4) The root-mean-square (rms) value of an ac voltage described by (1) is proportional to the integral of the square of the instantaneous voltage over a period of the voltage, average power delivered to a load by such a voltage is the ratio of the square of rms voltage to the resistance of the load.An ac power controller is a unit that can vary the rms value of the voltage across a load while keeping the frequency constant. Methods that are often used in power control are ON/OFF control, Phase-angle control and pulse-width modulation ac chopper control [2].Power electronic components such as Triac and Thyristors are often used to delay the firing angle in a wave; this causes only part of the wave to be outputted to the load. The firing angle ‘α’ of these devices is the angle in a half-cycle of a sinusoidal signal at which they begin to conduct; is usually the object of control. The remaining angle of the cycle in which the device conducts is the conduction angle ‘ϕ’. These angles are complimentary.In analogue circuits the firing angle is usually controlled through an RC network in which a capacitor charges through a resistor and discharges through the gate of the Triac, such a network can only change the firing angle between 0° and 90°, but with operational amplifiers and more sophisticated circuits, the firing angle can be changed from 0° to 180° in both cycles of the ac voltage [4]. A digital circuit that can be used to provide control of ac power: precision ac power control was reported by [3]; apart from the complexity of the circuit, the system is not purely electronic.A microelectronics system has a microprocessor in its International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 www.ijert.org IJERTV3IS110029 (This work is licensed under a Creative Commons Attribution 4.0 International License.) Vol. 3 Issue 11, November-2014 45